The objective is to develop a mechanical, currentless and optical switch that can be used wherever access to key infrastructures needs to be secured respectively monitored. In addition to telecommunication shafts, such key structures include shafts and access doors of gas, water and power supply that must be secured against unauthorized access. Such shafts are often located underground and are closed by a shaft cover. The multi-purpose switch should be installed in the shafts to monitor the opening status of the respective shaft cover.
In addition, the switch should also make it possible to monitor the accesses to technical or high-security rooms or containers and also, in combination with a float valve, to indicate the flooding of a shaft or a building level for instance.
U.S. Pat. No. 7,109,873 B2 mentions a switch that can be used to detect the opening of a shaft cover. In one embodiment of this switch, a movable pin in the switch is pre-tensioned with a spring and presses from the inside against the closed shaft cover. When the shaft cover is opened, the pin moves and bends an optical conductor (glass fiber) that is fixed on the pin. Alternatively, magnets can be used to trigger the bending of the optical conductor (glass fiber) when the shaft cover is opened. To that end, it is necessary to fix a first magnet on the shaft cover, this magnet attracts a second magnet, which is pre-tensioned with a spring, when the shaft cover is closed. When opening the shaft cover, the pre-tension is neutralized and the switch triggers. When the switch is triggered, a glass fiber is deflected and bent by a taper pin that is coupled with the second magnet.
The bending (buckling) of the optical conductor is recorded by a measuring device on the basis of the optical time-domain reflectometry (OTDR). For this purpose, an OTDR measuring device is connected to the optical conductor, which sends per measurement a light pulse through the optical conductor and measures the intensity of the back-scattered light over time. If a bending in the optical conductor exists, the total reflection of light is compensated in the optical conductor and light exits from the optical conductor. The measurement of the back-scattered light over time cannot only be used to determine the magnitude, but also the location at which the light intensity is lost due to temporary damping. Since the measurement is a pulsed measurement that sends light pulses at different time intervals and since the glass fiber requires a certain time to go back to the initial position, the switching status of the switch cannot be checked in the time between two light pulses. If the switch is triggered only for a very short time, then the triggering is not reliably detected.